United Nations: The UN special investigator on counter-terrorism and human rights has accused Republican presidential nominee Donald Trump of peddling "lies and xenophobia" by claiming a link between Syrian refugees and Islamic State extremists.

British human rights lawyer Ben Emmerson told a press conference "there is no evidence that terrorist groups take advantage of refugee flows to carry out acts of terrorism, or that refugees are somehow more prone to radicalisation than others."

"Almost without exception, refugees and migrants do not pose a risk of terrorism," he said.

"They are, in fact, at risk of fleeing the areas where terrorist groups are most active. And irresponsible statements of the kind made by Mr Trump on Wednesday do nothing but fuel prejudice and stigma."

A file photo of Donald Trump. AP

Emmerson was responding to Trump's statement during the final presidential debate that Democratic opponent Hillary Clinton is "taking in tens of thousands of Syrian refugees who probably, in many cases, not probably, who are definitely, in many cases, ISIS-aligned," using another acronym for IS.

He said other leaders in a number European countries and elsewhere have implied that routes used by migrants "are capable of being misused by returning foreign fighters and members of ISIS."

"The reality is that the evidence shows that there is almost no statistical support for that," Emmerson said. "The reason I single out Donald Trump is ... because I think he went further than any other leader has gone by asserting that there was definitely a link between substantial numbers of Syrian refugees into the United States and ISIS, and that is nothing more or less than a xenophobic lie."

He said that in 2015, more than 65 million people were estimated to be displaced including 12 million or 1 million a month forced to leave their homes that year.

"The link between displacement of people from their homes and risks to national security in the countries in which they seek refuge has been irresponsibly and misleadingly overblown in many states, with fears of terrorism being used to stoke public fears in the refugee crisis," Emmerson said.

He said Trump's comments are "one of the clearest examples" and they involve "the peddling of lies and xenophobia."

Emmerson spoke to reporters after presenting his report on the impact of counter-terrorism measures on the human rights of migrants and refugees to the UN General Assembly.